Interpretive Summary: Leptospirosis affects most mammalian species and is one of the most common zoonotic diseases known. An outbreak of leptospirosis in 2004 along the Pacific Coast of North America resulted in a significant number of deaths among several different marine mammal species, including several which are endangered or threatened with extinction. California sea lions were infected and died during this outbreak throughout a region extending from California to British Columbia. This is consistent with propagation of new infections in the sea lion population during their seasonal migration. The results of this study help show how animal migration may lead to the dissemination of leptospirosis. Analysis of the bacterial isolates may shed light on the original source of infection and help lead to intervention strategies that will improve the health of marine mammal populations.

Technical Abstract:
Leptospirosis is one of the most widespread bacterial zoonoses in the world, and affects both terrestrial and marine mammals. Little information is available describing the geographical spread of leptospirosis, but it is logical to assume that animal migrations contribute to this process. There have been several documented outbreaks of leptospirosis among California sea lions (Zalophus californianus) since 1970. During the most recent epizootic event in 2004, infected sea lions were identified from a region extending from the breeding grounds off the southern California coast to British Columbia. The occurrence of infected animals coincided with the seasonal migration of male sea lions to their winter feeding grounds. Many beached animals were found along fresh water estuaries, increasing the likelihood of contact with terrestrial animals, and may contribute to dissemination of leptospirosis across broad geographical areas.